In an interview Thursday with The Wall Street Journal — his first since the deal was announced — Mr. Koum said Mr. Zuckerberg’s offer to keep WhatsApp operating as a standalone business with no ads was a key turning point in convincing the startup to sell. Rather than be absorbed into Facebook’s business, Mr. Koum sees WhatsApp functioning as a separate arm that will help the social network diversify its revenue streams as it grows.

Below are edited excerpts of the interview with Mr. Koum:

WSJ: What have the past 24 hours been like?

Mr. Koum: Pretty amazing. To say that we’re excited and humbled at the same time would be an understatement.

Mark [Zuckerberg] stopped by the office yesterday after the announcement. We thought it would be good for him to stop by the office and answer any questions people might have and reassure the team that we will stay independent, we will stay autonomous. He talked about how the last thing we want to do, given how much we have achieved, is get in the way of our growth.

WSJ: You have said repeatedly that you want to build an independent business. How did Mr. Zuckerberg finally change your mind?

Mr. Koum: It was very difficult for him to find a good way that would make us be interested in a conversation. But as we got to know each other, we got to respect each other more and more over the years. Once he made it very clear that they are committed to not make changes to the user experience by introducing ads or cluttering the user experience, it became very interesting for us to have a conversation.

Mark proposed something very interesting two weeks ago, where he positioned it more as a partnership. He positioned it more as WhatsApp will stay completely independent and autonomous.

WSJ: What do you have in common with Mr. Zuckerberg?

Mr. Koum: We have a shared mission of connecting the world and making it more open. Obviously, what we do is communication, which is part of connecting the world. But we also have a very long-term vision. Sometimes we talk about where mobile and communication will be five years from now, 10 years from now. The way he thinks about it is almost the same way that I think about it.

WSJ: Where do you think WhatsApp fits into Facebook’s broader plans for growth?

Mr. Koum: Mark deserves a lot of credit for realizing that it’s good to have different business models in your company. If you look at firms like General Electric or other large companies, they don’t just do one thing, they do many different things to generate sources of revenue. I was actually thinking just earlier today how in the late ’90s Porsche only used to make sports cars. Now they make SUVs and sedans and all kinds of different models because it’s good to have diversification in your business.

We’re actually making this business stronger by bringing in yet another type of revenue stream in the future.

WSJ: You are taking a seat on Facebook’s board. Was that your idea or Mr. Zuckerberg’s?

Mr. Koum: It was all his idea.

WSJ: Now that you have a board seat, do you want to push for changes at this company and be a voice for protecting privacy and user data?

Mr. Koum: To be fair to Mark, I don’t understand a whole lot about Facebook’s internal metrics and statistics and how the company works and different ways to generate revenue. I told him that. He still insisted that I join the board and help him with decisions, which is very flattering.

I think I will always have my opinions and philosophy, which I will share with him. But for the next few months or few years my focus will be on WhatsApp.

WSJ: What will determine the success of this deal in the coming years?

Mr. Koum: For us, it’s about staying independent. It’s about staying autonomous, as was discussed and announced during the deal. And it’s also about growing. We still have a mission. We still need to get to a billion users and then we need to get to two billion users. And we won’t stop until every single person on the planet has an affordable and reliable way to communicate with their friends and loved ones.